In most studies for physics, chemistry and material fields, it is very important to measure optical properties of materials and a thickness of a thin film. Particularly, in recent semiconductor and display industries, it is very important to define and control thicknesses and optical properties of PR, ITO, and gate oxide films between fabricating processes in an aspect of an yield of product and improvement of performance.
Various principles have currently been known to measure optical properties of materials and a thickness of a thin film. Among them, an ellipsometry is much improved in its performance with development of a light source, an optical detector and a computer and is largely increased in its application with increasing of processes using thin films and surfaces.
The ellipsometry is divided into a reflection type and a transmission type and the reflection type ellipsometry, which analyzes a polarization of light reflected from a surface of a sample with an incidence angle, is widely used. The reflection type ellipsometry may mainly be used for obtaining optical properties of the sample such as a refractive index and an extinction coefficient by measuring variation in polarization of light reflected by the sample and may also be used for obtaining properties such as surface state of the sample.
A plane which is perpendicular to a surface of the sample and is on a light path is defined as an incidence surface. A case that direction of an electric field vector of light is perpendicular to the incidence surface is referred to s-wave and a case that direction of the electric field vector of the light is on the incidence surface is referred to p-wave.
If a structure of a sample and optical properties, a thickness of a thin film and an incidence angle of element materials of the sample are given, reflection coefficients of the s-wave and the p-wave respectively rs and rp in a specific wavelength are calculated by the following equations.
            r      p        =                            E          rp                          E          ip                    =                                              r            p                                    ⁢                  ⅇ                      ⅈδ            p                                          r      s        =                            E          rs                          E          is                    =                                              r            s                                    ⁢                  ⅇ                      ⅈδ            s                              wherein, |rp(x)| is a ratio of an electric field Erp(s) of an reflected wave to an electric field Eip(s) of an incident wave. In addition, δp(s) is a phase variation due to a reflection. In the ellipsometry, a complex reflection coefficient ratio ρ, which is a reflection coefficient ratio of the p-wave to the s-wave, is defined as follows:
  ρ  =                    r        p                    r        s              =                                                r            p                                r            s                                      ⁢              ⅇ                  ⅈ          ⁡                      (                                          δ                p                            -                              δ                s                                      )                              From this, ellipsometric angles ψ and Δ are defined as follows:
      ψ    =                  tan                  -          1                    ⁡              (                                                      r              p                                      r              s                                                )                  Δ    =                  δ        p            -              δ        s            wherein, tan ψ is a magnitude ratio of reflection coefficient and Δ is a phase difference between the p-wave and the s-wave after the p-wave and the s-wave incident with the same phase are reflected
Ellipsometers having various functions and structures are disclosed in prior patents and papers. As a typical ellipsometer, there are a null ellipsometer which finds a null point by controlling a linear polarizer and a compensator, a rotating-polarizer ellipsometer in which a linear polarizer of a light source part module rotates at a constant speed, a rotating-analyzer ellipsometer in which a linear polarizer of a light receiving part module rotates at a constant speed or a rotating-compensator ellipsometer in which a compensator of a light receiving part module rotates at a constant speed.
FIG. 1 shows a rotating-polarizer ellipsometer which is one of the widely used ellipsometers. The rotating-polarizer ellipsometer 300 is provided with a light source part module 330, in which a linear polarizer rotates at a constant speed by a step motor or a DC motor, so as to polarize light 320 emitted from a light source 310. The light source part module 330 may consist of a collimating lens, a linear polarizer and so on. The light passed through the light source part module 330 is linearly polarized and the polarization axis rotates at the same speed as the driving motor. The light reflected on a surface of a sample is received by a light receiving part module 350 after its polarization is varied due to optical properties of the sample. The light receiving part module 350 may consist of a linear polarizer, a compensator and so on and allows a filtered specific polarization component to transmit. An optical detector 360 detects an intensity of the light passed through the light receiving part module 350 and incident thereto as an electric signal such as voltage or current. The intensity signal of the detected light is processed in a processing device 370 in combination with information of an azimuth angle of polarization axis of the polarizer in the light source part module 330 and information of an azimuth angle of polarization axis of the polarizer in light receiving part module 350.
In the above mentioned ellipsometer, as the light source, a white light source such as a tungsten halogen lamp, a xenon lamp, etc. and a monochromatic light source such as a laser, etc. may be used. The light source part module 330 or the light receiving part module 350 may be added with a spectroscope when a white light source is used. Besides the above mentioned ellipsometer, various ellipsometers have been developed and varied structures may be obtained from the basic structure through adding, removing or driving a compensator, a phase modulation device, etc.
In the case of a rotating-polarizer ellipsometer in FIG. 1, the light source part module 320 and the light receiving part 350 consist of a linear polarizer alone, respectively and the azimuth angle of polarization axis of the polarizer of the light source part module 330 rotates at a uniform angular speed ω, a voltage signal V(t) detected by the optical detector is a function of time t and may be expressed by the following equation.V(t)=Vav+α cos(2ωt)+b sin(2ωt)wherein, Vav is a time average value of a voltage and a and b are Fourier coefficients for an angular frequency of 2ω. Fourier coefficients normalized by dividing both members of the equation by Vav are as follows:
                    α        =                              a                          V              av                                =                                                                      tan                  2                                ⁡                                  (                  ψ                  )                                            -                                                tan                  2                                ⁡                                  (                  A                  )                                                                                                      tan                  2                                ⁡                                  (                  ψ                  )                                            +                                                tan                  2                                ⁡                                  (                  A                  )                                                                                            β        =                              b                          V              av                                =                                    2              ⁢                              tan                ⁡                                  (                  ψ                  )                                            ⁢              cos              ⁢                                                          ⁢                              (                Δ                )                            ⁢                              tan                ⁡                                  (                  A                  )                                                                                                      tan                  2                                ⁡                                  (                  ψ                  )                                            +                                                tan                  2                                ⁡                                  (                  A                  )                                                                        wherein, A is the azimuth angle of polarization axis of the polarizer in the light receiving part module 350. Putting in order again the equation, the ellipsometric angle is obtained using the following relation equation.
      ψ    =                  tan                  -          1                    ⁡              (                                                            1                +                α                                            1                -                α                                      ⁢            tan            ⁢                                                  ⁢            A                          )                  Δ    =                  cos                  -          1                    ⁡              (                  β                                    1              -                              α                2                                                    )            
The resultant ellipsometric angles ψ and Δ derived from the above can be expressed with Fresnel reflection coefficients rp and rs for p polarized light and s polarized light and thus physical quantities of a sample can be obtained in a process of comparing them with values of ψ and Δ calculated using model equations suitable to optical properties of the sample 340.
The ellipsometry is a technology belonging to an optical metrology among methods for measuring optical properties of a material and a thickness of a thin film and has a superiority of being sensitive to an ultra thin film compared to similar technologies such as reflectometry, interferometry, etc.
Since a rotating-analyzer ellipsometer, a rotating-polarizer ellipsometer and a rotating-compensator ellipsometer which are generally and widely used have, due to its structure, a driving part which rotates mechanically by a step motor or a DC motor, a measuring time is limited according to a speed of the motor and signal noise due to vibration generated from mechanical rotation exists always.
In a phase modulation type which is another type of an ellipsometer, the measuring time is limited by a phase modulation frequency of a photoelastic modulator. The rotation speed of the polarizer is about 10˜100 Hz in the rotating-polarizer ellipsometer and the phase modulation frequency is about 50 kHz in the phase modulation type ellipsometer and thus the phase modulation type ellipsometer is relatively suitable to a high speed measurement, but a thermostat is required since the photoelastic modulator has a temperature dependency and a light wavelength dependency and there is a difficulty of correcting optical properties of the photoelastic modulator for wavelengths of used light.
Structures similar to a structure of the focused-beam ellipsometer according to the present invention are disclosed in U.S. Pat. Nos. 4,999,014, 5,042,951 and 5,181,080. Such prior patents have intended to improve an accuracy of a measurement result for a nano thin film. However, in the focused-beam ellipsometry of the prior patent, there has necessarily been a limitation in improvement of measurement accuracy for ψ and Δ since ψ and Δ are calculated by taking a part of reflected light formed by a structure of the focused-beam ellipsometer.